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bit_reader.h
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bit_reader.h
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/* Copyright 2013 Google Inc. All Rights Reserved.
Distributed under MIT license.
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/
/* Bit reading helpers */
#ifndef BROTLI_DEC_BIT_READER_H_
#define BROTLI_DEC_BIT_READER_H_
#include <string.h> /* memcpy */
#include "../common/constants.h"
#include "../common/platform.h"
#include <brotli/types.h>
#if defined(__cplusplus) || defined(c_plusplus)
extern "C" {
#endif
#define BROTLI_SHORT_FILL_BIT_WINDOW_READ (sizeof(brotli_reg_t) >> 1)
BROTLI_INTERNAL extern const uint32_t kBrotliBitMask[33];
static BROTLI_INLINE uint32_t BitMask(uint32_t n) {
if (BROTLI_IS_CONSTANT(n) || BROTLI_HAS_UBFX) {
/* Masking with this expression turns to a single
"Unsigned Bit Field Extract" UBFX instruction on ARM. */
return ~((0xFFFFFFFFu) << n);
} else {
return kBrotliBitMask[n];
}
}
typedef struct {
brotli_reg_t val_; /* pre-fetched bits */
uint32_t bit_pos_; /* current bit-reading position in val_ */
const uint8_t* next_in; /* the byte we're reading from */
size_t avail_in;
} BrotliBitReader;
typedef struct {
brotli_reg_t val_;
uint32_t bit_pos_;
const uint8_t* next_in;
size_t avail_in;
} BrotliBitReaderState;
/* Initializes the BrotliBitReader fields. */
BROTLI_INTERNAL void BrotliInitBitReader(BrotliBitReader* const br);
/* Ensures that accumulator is not empty.
May consume up to sizeof(brotli_reg_t) - 1 bytes of input.
Returns BROTLI_FALSE if data is required but there is no input available.
For BROTLI_ALIGNED_READ this function also prepares bit reader for aligned
reading. */
BROTLI_INTERNAL BROTLI_BOOL BrotliWarmupBitReader(BrotliBitReader* const br);
/* Fallback for BrotliSafeReadBits32. Extracted as noninlined method to unburden
the main code-path. Never called for RFC brotli streams, required only for
"large-window" mode and other extensions. */
BROTLI_INTERNAL BROTLI_NOINLINE BROTLI_BOOL BrotliSafeReadBits32Slow(
BrotliBitReader* const br, uint32_t n_bits, uint32_t* val);
static BROTLI_INLINE void BrotliBitReaderSaveState(
BrotliBitReader* const from, BrotliBitReaderState* to) {
to->val_ = from->val_;
to->bit_pos_ = from->bit_pos_;
to->next_in = from->next_in;
to->avail_in = from->avail_in;
}
static BROTLI_INLINE void BrotliBitReaderRestoreState(
BrotliBitReader* const to, BrotliBitReaderState* from) {
to->val_ = from->val_;
to->bit_pos_ = from->bit_pos_;
to->next_in = from->next_in;
to->avail_in = from->avail_in;
}
static BROTLI_INLINE uint32_t BrotliGetAvailableBits(
const BrotliBitReader* br) {
return (BROTLI_64_BITS ? 64 : 32) - br->bit_pos_;
}
/* Returns amount of unread bytes the bit reader still has buffered from the
BrotliInput, including whole bytes in br->val_. Result is capped with
maximal ring-buffer size (larger number won't be utilized anyway). */
static BROTLI_INLINE size_t BrotliGetRemainingBytes(BrotliBitReader* br) {
static const size_t kCap = (size_t)1 << BROTLI_LARGE_MAX_WBITS;
if (br->avail_in > kCap) return kCap;
return br->avail_in + (BrotliGetAvailableBits(br) >> 3);
}
/* Checks if there is at least |num| bytes left in the input ring-buffer
(excluding the bits remaining in br->val_). */
static BROTLI_INLINE BROTLI_BOOL BrotliCheckInputAmount(
BrotliBitReader* const br, size_t num) {
return TO_BROTLI_BOOL(br->avail_in >= num);
}
/* Guarantees that there are at least |n_bits| + 1 bits in accumulator.
Precondition: accumulator contains at least 1 bit.
|n_bits| should be in the range [1..24] for regular build. For portable
non-64-bit little-endian build only 16 bits are safe to request. */
static BROTLI_INLINE void BrotliFillBitWindow(
BrotliBitReader* const br, uint32_t n_bits) {
#if (BROTLI_64_BITS)
if (!BROTLI_ALIGNED_READ && BROTLI_IS_CONSTANT(n_bits) && (n_bits <= 8)) {
if (br->bit_pos_ >= 56) {
br->val_ >>= 56;
br->bit_pos_ ^= 56; /* here same as -= 56 because of the if condition */
br->val_ |= BROTLI_UNALIGNED_LOAD64LE(br->next_in) << 8;
br->avail_in -= 7;
br->next_in += 7;
}
} else if (
!BROTLI_ALIGNED_READ && BROTLI_IS_CONSTANT(n_bits) && (n_bits <= 16)) {
if (br->bit_pos_ >= 48) {
br->val_ >>= 48;
br->bit_pos_ ^= 48; /* here same as -= 48 because of the if condition */
br->val_ |= BROTLI_UNALIGNED_LOAD64LE(br->next_in) << 16;
br->avail_in -= 6;
br->next_in += 6;
}
} else {
if (br->bit_pos_ >= 32) {
br->val_ >>= 32;
br->bit_pos_ ^= 32; /* here same as -= 32 because of the if condition */
br->val_ |= ((uint64_t)BROTLI_UNALIGNED_LOAD32LE(br->next_in)) << 32;
br->avail_in -= BROTLI_SHORT_FILL_BIT_WINDOW_READ;
br->next_in += BROTLI_SHORT_FILL_BIT_WINDOW_READ;
}
}
#else
if (!BROTLI_ALIGNED_READ && BROTLI_IS_CONSTANT(n_bits) && (n_bits <= 8)) {
if (br->bit_pos_ >= 24) {
br->val_ >>= 24;
br->bit_pos_ ^= 24; /* here same as -= 24 because of the if condition */
br->val_ |= BROTLI_UNALIGNED_LOAD32LE(br->next_in) << 8;
br->avail_in -= 3;
br->next_in += 3;
}
} else {
if (br->bit_pos_ >= 16) {
br->val_ >>= 16;
br->bit_pos_ ^= 16; /* here same as -= 16 because of the if condition */
br->val_ |= ((uint32_t)BROTLI_UNALIGNED_LOAD16LE(br->next_in)) << 16;
br->avail_in -= BROTLI_SHORT_FILL_BIT_WINDOW_READ;
br->next_in += BROTLI_SHORT_FILL_BIT_WINDOW_READ;
}
}
#endif
}
/* Mostly like BrotliFillBitWindow, but guarantees only 16 bits and reads no
more than BROTLI_SHORT_FILL_BIT_WINDOW_READ bytes of input. */
static BROTLI_INLINE void BrotliFillBitWindow16(BrotliBitReader* const br) {
BrotliFillBitWindow(br, 17);
}
/* Tries to pull one byte of input to accumulator.
Returns BROTLI_FALSE if there is no input available. */
static BROTLI_INLINE BROTLI_BOOL BrotliPullByte(BrotliBitReader* const br) {
if (br->avail_in == 0) {
return BROTLI_FALSE;
}
br->val_ >>= 8;
#if (BROTLI_64_BITS)
br->val_ |= ((uint64_t)*br->next_in) << 56;
#else
br->val_ |= ((uint32_t)*br->next_in) << 24;
#endif
br->bit_pos_ -= 8;
--br->avail_in;
++br->next_in;
return BROTLI_TRUE;
}
/* Returns currently available bits.
The number of valid bits could be calculated by BrotliGetAvailableBits. */
static BROTLI_INLINE brotli_reg_t BrotliGetBitsUnmasked(
BrotliBitReader* const br) {
return br->val_ >> br->bit_pos_;
}
/* Like BrotliGetBits, but does not mask the result.
The result contains at least 16 valid bits. */
static BROTLI_INLINE uint32_t BrotliGet16BitsUnmasked(
BrotliBitReader* const br) {
BrotliFillBitWindow(br, 16);
return (uint32_t)BrotliGetBitsUnmasked(br);
}
/* Returns the specified number of bits from |br| without advancing bit
position. */
static BROTLI_INLINE uint32_t BrotliGetBits(
BrotliBitReader* const br, uint32_t n_bits) {
BrotliFillBitWindow(br, n_bits);
return (uint32_t)BrotliGetBitsUnmasked(br) & BitMask(n_bits);
}
/* Tries to peek the specified amount of bits. Returns BROTLI_FALSE, if there
is not enough input. */
static BROTLI_INLINE BROTLI_BOOL BrotliSafeGetBits(
BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
while (BrotliGetAvailableBits(br) < n_bits) {
if (!BrotliPullByte(br)) {
return BROTLI_FALSE;
}
}
*val = (uint32_t)BrotliGetBitsUnmasked(br) & BitMask(n_bits);
return BROTLI_TRUE;
}
/* Advances the bit pos by |n_bits|. */
static BROTLI_INLINE void BrotliDropBits(
BrotliBitReader* const br, uint32_t n_bits) {
br->bit_pos_ += n_bits;
}
static BROTLI_INLINE void BrotliBitReaderUnload(BrotliBitReader* br) {
uint32_t unused_bytes = BrotliGetAvailableBits(br) >> 3;
uint32_t unused_bits = unused_bytes << 3;
br->avail_in += unused_bytes;
br->next_in -= unused_bytes;
if (unused_bits == sizeof(br->val_) << 3) {
br->val_ = 0;
} else {
br->val_ <<= unused_bits;
}
br->bit_pos_ += unused_bits;
}
/* Reads the specified number of bits from |br| and advances the bit pos.
Precondition: accumulator MUST contain at least |n_bits|. */
static BROTLI_INLINE void BrotliTakeBits(
BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
*val = (uint32_t)BrotliGetBitsUnmasked(br) & BitMask(n_bits);
BROTLI_LOG(("[BrotliTakeBits] %d %d %d val: %6x\n",
(int)br->avail_in, (int)br->bit_pos_, (int)n_bits, (int)*val));
BrotliDropBits(br, n_bits);
}
/* Reads the specified number of bits from |br| and advances the bit pos.
Assumes that there is enough input to perform BrotliFillBitWindow.
Up to 24 bits are allowed to be requested from this method. */
static BROTLI_INLINE uint32_t BrotliReadBits24(
BrotliBitReader* const br, uint32_t n_bits) {
BROTLI_DCHECK(n_bits <= 24);
if (BROTLI_64_BITS || (n_bits <= 16)) {
uint32_t val;
BrotliFillBitWindow(br, n_bits);
BrotliTakeBits(br, n_bits, &val);
return val;
} else {
uint32_t low_val;
uint32_t high_val;
BrotliFillBitWindow(br, 16);
BrotliTakeBits(br, 16, &low_val);
BrotliFillBitWindow(br, 8);
BrotliTakeBits(br, n_bits - 16, &high_val);
return low_val | (high_val << 16);
}
}
/* Same as BrotliReadBits24, but allows reading up to 32 bits. */
static BROTLI_INLINE uint32_t BrotliReadBits32(
BrotliBitReader* const br, uint32_t n_bits) {
BROTLI_DCHECK(n_bits <= 32);
if (BROTLI_64_BITS || (n_bits <= 16)) {
uint32_t val;
BrotliFillBitWindow(br, n_bits);
BrotliTakeBits(br, n_bits, &val);
return val;
} else {
uint32_t low_val;
uint32_t high_val;
BrotliFillBitWindow(br, 16);
BrotliTakeBits(br, 16, &low_val);
BrotliFillBitWindow(br, 16);
BrotliTakeBits(br, n_bits - 16, &high_val);
return low_val | (high_val << 16);
}
}
/* Tries to read the specified amount of bits. Returns BROTLI_FALSE, if there
is not enough input. |n_bits| MUST be positive.
Up to 24 bits are allowed to be requested from this method. */
static BROTLI_INLINE BROTLI_BOOL BrotliSafeReadBits(
BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
BROTLI_DCHECK(n_bits <= 24);
while (BrotliGetAvailableBits(br) < n_bits) {
if (!BrotliPullByte(br)) {
return BROTLI_FALSE;
}
}
BrotliTakeBits(br, n_bits, val);
return BROTLI_TRUE;
}
/* Same as BrotliSafeReadBits, but allows reading up to 32 bits. */
static BROTLI_INLINE BROTLI_BOOL BrotliSafeReadBits32(
BrotliBitReader* const br, uint32_t n_bits, uint32_t* val) {
BROTLI_DCHECK(n_bits <= 32);
if (BROTLI_64_BITS || (n_bits <= 24)) {
while (BrotliGetAvailableBits(br) < n_bits) {
if (!BrotliPullByte(br)) {
return BROTLI_FALSE;
}
}
BrotliTakeBits(br, n_bits, val);
return BROTLI_TRUE;
} else {
return BrotliSafeReadBits32Slow(br, n_bits, val);
}
}
/* Advances the bit reader position to the next byte boundary and verifies
that any skipped bits are set to zero. */
static BROTLI_INLINE BROTLI_BOOL BrotliJumpToByteBoundary(BrotliBitReader* br) {
uint32_t pad_bits_count = BrotliGetAvailableBits(br) & 0x7;
uint32_t pad_bits = 0;
if (pad_bits_count != 0) {
BrotliTakeBits(br, pad_bits_count, &pad_bits);
}
return TO_BROTLI_BOOL(pad_bits == 0);
}
/* Copies remaining input bytes stored in the bit reader to the output. Value
|num| may not be larger than BrotliGetRemainingBytes. The bit reader must be
warmed up again after this. */
static BROTLI_INLINE void BrotliCopyBytes(uint8_t* dest,
BrotliBitReader* br, size_t num) {
while (BrotliGetAvailableBits(br) >= 8 && num > 0) {
*dest = (uint8_t)BrotliGetBitsUnmasked(br);
BrotliDropBits(br, 8);
++dest;
--num;
}
memcpy(dest, br->next_in, num);
br->avail_in -= num;
br->next_in += num;
}
#if defined(__cplusplus) || defined(c_plusplus)
} /* extern "C" */
#endif
#endif /* BROTLI_DEC_BIT_READER_H_ */